1. Field of the Invention
This invention relates to electric heating elements and in particular relates to the sealing of an electric heating wire within the tubular metal sheath of the element.
2. Description of the Prior Art
Metal sheathed electric heaters are well known in the art and primarily consist of an electrical resistance wire which acts as the heater for the element. The resistance wire is generally disposed along the axis of the tubular sheath and is electrically insulated therefrom by an insulating filler which is packed into the surrounding space around the wire. This prevents relative movement between the wire and the sheath and enables heat generated in the wire to be transmitted to the sheath.
In order to prevent deterioration of the filler, it has been essential to seal the ends of the sheath to prevent the ingress of air and moisture. It is known to seal the ends of the sheath in a number of ways; a seat of suitable insulating material may be secured in the site and, if necessary, providing a heat resistant insulating material such as an epoxy resin between the seat and the inner filler. Alternatively, a seat may not be required but instead the sheath ends may be sealed directly with a heat-resistant insulating material such as a silicone varnish, glass seal, silicone rubber or an epoxy resin.
However a residual problem remains in that, over extended use and/or storage of the heating elements, air and moisture still find their way through the seals thereby still resulting in deterioration in the insulating property of the filler.